Backlinks are one of the most important factors that determine the chance a given website will successfully place high in Google's generated SERPs. So long as the links pointing to the site do not feature the "nofollow" element, Google will consider the existence of those links to be a strong indication that awareness of the site has spread on account of it being a viable authority on its subject matter.
There are primarily two environments inside a given external website that are appropriate for backlinks to appear within. The first is the main space of the site, where content posted by the author of the external website can include backlinks for the other site's benefit; this often happens on a blog feature maintained on the website where the author posts content updates regularly. The owner of a site can arrange a deal with another site's author to write a guest blog post on that feature. Likewise, the owner can make reasonable posts on the external website's forum to directly spread awareness of their site. This is the second environment in which "dofollow" backlinks are likely to appear because community discussions that take place within forums commonly result in members voluntarily bringing up other resources and sharing links to them.
There are many "black hat" methods of getting backlinks to populate the Internet without the site owner compelling their emergence through organic means. The algorithm Google employs to disavow links that appear to have been created to artificially increase a website's prominence is informally called "Google Penguin," which is regularly updated to catch backlinks resulting from new innovations in black hat trends. Collections of web pages that exist solely to point to the same website are called link farms, and the most recent version of the Penguin algorithm has made link farms more ineffective at increasing websites' prominence in SERPs than ever. So long as Google does not designate a website with an active forum to be a link farm, backlinks included within posts made on that forum will be significantly effective at improving the linked website's search rankings. For more information click here https://www.reddit.com/r/SEO/comments/c6vnc0/areforumdofollowlinksdoesanygoodforseo/.